In the present work, the density functional theory formalism was used to study the reaction between coadsorbed CO and NH species, the last coming from ammonia decomposition, to form NCO over Rh-2. The optimized structures for each stage of the reaction were obtained and a mu(2)(N,C)-CONH precursor state towards the NCO formation was found. The transition states structures were also modeled, which allowed to analyze the reaction pathway. When CO and NH species approach each other, they form easily the precursor state. From this stage, it decomposes to NCO and H surpassing an activation barrier of 1.17 eV. This picture is in agreement with the infrared spectroscopy results obtained in the reaction of CO with NH3 over Rh/Al2O3 catalysts: a gradual decreasing of the feature assigned to the precursor state with a parallel increasing of the characteristic band due to the NCO asymmetric stretching mode. (C) 2004 Elsevier B.V. All rights reserved.